Generally, semiconductor memory devices are mainly divided into volatile memory and non-volatile memory according to a driving method. Dynamic Random Access Memory (DRAM), which is a representative volatile memory device, includes a single transistor and a single capacitor. Such DRAM is advantageous in that the operating speed thereof is very high, and the degree of integration is also very high, thus enabling large-capacity memory to be implemented. However, DRAM is disadvantageous in that since a capacitor must be continuously recharged with electric charges, power consumption is high, and, in particular, in that DRAM is volatile memory in which all data stored therein is erased when the supply of power to the DRAM is interrupted.
Unlike this, a non-volatile memory device is advantageous in that even if power is turned off, data stored in the memory can be retained for a long period of time. A representative non-volatile memory device includes flash memory. However, flash memory is disadvantageous in that operating voltage is high, and operating speed is much lower than that of DRAM.
Accordingly, extensive research has been conducted into new memory devices capable of solving the disadvantages of DRAM which is a volatile memory device and flash memory which is a non-volatile memory device, and maximizing the advantages thereof, for example, a Phase change Random Access Memory (PRAM) device that uses a resistance variation when a phase change between materials occurs, Magnetic Random Access Memory (MRAM) that uses a variation in the giant magneto-resistance of a ferromagnetic body, Ferroelectric Random Access Memory (FRAM) that uses the polarization of a ferroelectric body, and Resistive Random Access Memory (RRAM or ReRAM) that uses the resistance variation characteristics of a material.
Among the above memory devices, RRAM generally has a Metal/Metal Oxide (Insulator)/Metal (MIM) structure based on a metal oxide, and exhibits characteristics as a memory device in such a way that when a suitable electrical signal is applied to a metal oxide, the state of the metal oxide changes from a High Resistance State (HRS or OFF state) to a Low Resistance State (LRS or ON state), or vice versa.
Resistance can be classified into Current Controlled Negative Differential Resistance (CCNR) and Voltage Controlled Negative Differential Resistance (VCNR) according to an electrical method of implementing ON/OFF switching memory characteristics. In particular, in the case of VCNR, current exhibits the characteristics of, as voltage increases, changing from a high current state to a low current state. By using a large difference between the resistances appearing at that time, memory characteristics can be implemented.
A lot of research into the switching characteristics of a metal oxide that has a resistance state changing with an applied voltage has been conducted for a long period of time, and, as a result, two switching models have been presented. First, a certain structural variation is caused in a metal oxide, and thus a high conductivity path having a resistance state differing from that of the original metal oxide is formed. This is a conducting filament model. According to this model, an electrode metal material is diffused or injected into a thin film by an electrical stress (typically called a forming process), or, alternatively, defective structures in the thin film are rearranged, and thus a conducting filament having very high conductivity is formed. This conducting filament exhibits switching characteristics, because of the repetition of a phenomenon in which the conducting filament is broken due to joule heating occurring in a local area and is re-formed due to factors, such as the internal temperature and external temperature of the thin film, an applied electric field, and space charges. Second, there is a switching model based on a large number of traps present in a metal oxide. Generally, the metal oxide includes a large number of traps related to metal particles or oxygen particles. It is known that such a switching model exhibits switching characteristics in such a way that when the traps are charged with electric charges and the electric charges are discharged from the traps, band bending occurs in the interface between an electrode and a thin film, or, alternatively, a variation in an internal electric field is caused due to space charges.
Using these mechanisms, an RRAM device exhibits an operating speed (several tens of nsec) much higher than that of existing flash memory, and can be operated even at a low voltage (2 to 5 V or less) as in the case of DRAM. Further, such an RRAM device has advantages in that it enables fast reading-writing as in the case of SRAM, and has a simple memory device structure not only to reduce defects that may occur in a manufacturing process, but also to reduce manufacturing costs, thus enabling inexpensive memory devices to be manufactured. Furthermore, since the RRAM device is rarely influenced by cosmic radiation, electromagnetic waves, or the like, it can exhibit its own functions even in cosmic space, and memory performance does not deteriorate even if writing and erasure are repeated 1010 times or more. Thanks to these advantages, the RRAM device can be applied to all devices requiring storage media, and has characteristics suitable for the purpose of a memory device that has become implemented as, especially, a System-on-a-Chip (SoC) such as an embedded Integrated Circuit (IC).
In the prior art, a single crystal silicon substrate, a Silicon On Insulator (SOI) substrate, or the like has been used as the substrate of memory. Since memory that uses such a substrate is not transparent, it is impossible not only to manufacture a transparent electronic device that can be applied to transparent displays or the like, but also to directly apply electronic devices to transparent electronic products such as large-size transparent displays or the like because memory is influenced by the size of a substrate when being manufactured.